NEURITES OF PERIOD-EXPRESSING PDH CELLS IN THE FLYS OPTIC LOBE EXHIBIT CIRCADIAN OSCILLATIONS IN MORPHOLOGY

Circadian rhythms have been shown both in the expression of the period (per) gene in 'lateral neurons' and in cells of the outermost neuropi l, or lamina, of the fly's optic lobe. Some lateral neurons also exhib it PDH peptidelike immunoreactivity, arborizing widely throughout the optic lobe. Using confocal microscopy in the housefly, we analysed the size and spacing of PDH neurite varicosities, sites of possible pepti de release exhibiting circadian rhythmicity. During the subjective day in constant darkness, there were fewer, larger varicosities than duri ng subjective night. The endogenous rhythm was masked by the light exp osure that occurred under a day-night cycle and continuous light condi tions. Our findings indicate that PDH neurites convey circadian inform ation out from the pacemaker, where they could regulate the circadian rhythms that have been described in the lamina, possibly via cyclical release of their peptide.